1. Field of the Invention
The present invention is concerned generally with apparatus and methods for use in detecting and identifying the naturally occurring radioactive elements in earth formations traversed by a borehole. More particularly, the invention concerns methods and apparatus for use relative to detection and identification of elemental concentrations of silicon or calcium in earth formations in the vicinity of the borehole by analysis of spectra of naturally emitted gamma rays from elements contained in formations which also contain calcium or silicon. The technique is also used to detect iron in and around a borehole.
2. Description of the Prior Art
In recent years gamma ray spectroscopy of earth formations in the vicinity of a borehole has been made practical by the development by highly stable scintillation detectors which may be lowered into a borehole. The scintillation detector is responsive to the gamma ray spectrum of gamma rays impinging upon the scintillation crystal. At the present time, two commercial well logging services are available for detecting the natural gamma ray spectra produced by uranium, potassium, and thorium (or from their radioactive daughters) in earth formations in the vicinity of a borehole.
In the first of these commercially available services three energy ranges or windows centered about selected gamma ray emission peaks for naturally occurring gamma rays in the decay series of the aforementioned elements are selected. Gamma ray count rates in each of these three energy ranges are transmitted to the surface and processed by a technique called spectrum stripping wherein standard calibration spectra for each of the individual elements (made in standard boreholes) are applied to the measurements of count rates made in the energy ranges chosen to detect each of the three elements sought to be detected. So called "stripping constants" derived from the measurement of the standard spectra in standard boreholes for each of three elements are then applied to the measured spectrum in the unknown earth formations surrounding the borehole. An estimate of the percentage of the particular three elements desired to be detected is made based on the stripping technique.
In a second, slightly more sophisticated commercially available technique, (such as that described in U.S. Pat. No. 3,976,878 to Chevalier, et al issued Aug. 24, 1976) five energy ranges or windows are utilized for the measurement of the spectrum of natural gamma rays emitted by the earth formations in the vicinity of a borehole. The five energy range measurements used in this technique are employed in a least squares fitting scheme to determine the elemental concentrations of the three elements, again based on the spectrum of each of the individual elements of the three taken in standard boreholes. The use of the five windows gives an overdetermined set of equations (i.e. 5 equations in 3 unknowns) which are statistically enhanced by the use of the count rate data from the extra two energy windows to distinguish this technique from that described in the foregoing paragraph.
It is obvious in the logging operation that the logging tool must be capable of withstanding the temperatures and pressure encountered in a bore hole as well as the extremely high hydrostatic pressures which can be encountered. At the same time while requiring mechanical strength, the tool must possess suitable characteristics to transmit the gamma rays to be measured through the tool housing so that successful measurements may be made.
The present invention is concerned with the measurement of low energy natural gamma ray spectra emitted from downhole formations to provide formation lithology type and/or casing thickness. This information is derived by a measurement of the differences in the photoelectric absorption gamma ray cross-sections of the materials between the formation source and the detector in the logging tool. Because the photoelectric cross-sections are appreciable for most elements in and around a well borehole only at very low gamma ray energies (E=20-200 Kev), it is important that the tool housing be designed for maximum transmission of these low energy signals to the detector.